The present invention relates to an optical sensor and more specifically to an active optical sensor emitting a light beam in an area to be studied and analyzing the light sent back or transmitted by this area.
A more specific object of the present invention is to provide such a sensor that operates even when an area of a medium to be studied only sends back or transmits a very small portion of the received light.
Thus, the present invention especially applies to the detection of obstacles or interfaces in diffusing mediums, for example in organic mediums.
The present invention aims at the cases where the reemitted light is difficult to detect by conventional methods using for example chopper and synchronous detection systems.
The present invention provides reinjecting into a laser a light reemitted at a frequency shifted with respect to the laser emission frequency, by using a specific type of laser, that is, a class B laser that, when light is reinjected therein at a frequency shifted by a value substantially corresponding to a natural frequency or relaxation frequency of this laser, generates a modulated laser beam, the modulation amplitude corresponding to a factor on the order of 103 to 106 with respect to a conventional heterodyne detection.
More specifically, the present invention provides an optical sensor including a class B laser source emitting a beam at a first optical frequency; means interacting with the beam to provide a radiation at a modified optical frequency, the shifting between the first frequency and the modified frequency being close to a relaxation frequency of the laser; means for irradiating a selected area of a medium to be studied with the radiation at the modified frequency; means for injecting into the laser light sent back by the selected area; and means for detecting the disturbance brought to the laser emission by the reinjected light.
According to an embodiment of the present invention, the sensor includes means for scanning the area to be studied.
According to an embodiment of the present invention, the sensor includes means of frequency scanning of the radiation emitted by the laser.
According to an embodiment of the present invention, the irradiation means are formed of a lens.
According to an embodiment of the present invention, the detection means include a photodetector followed by a synchronous detection system.
According to an embodiment of the present invention, the means for providing a radiation at a modified optical frequency consist of a medium having an electro-optical or acousto-optical effect excited by an oscillator at a frequency corresponding to the difference between the modified optical frequency and the first optical frequency or one of its submultiples.
The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.